2012/03/05

Measuring the Acidity of a Solution


It is often useful and necessary to measure the acidity or basicity of a solution. There are a number of ways this could be described or reported, but in the context of the Bronsted-Lowry definitions of acids and bases, it is probably most convenient to monitor the concentration of H+(aq). The [H+] in a solution can be a very small number. Although modern pocket calculators can readily handle very small numbers, it's a little easier to describe these small concentrations using pH. pH expresses very small concentrations without having to use scientific notation and will be useful for a number of quantities.
pH = -log[H3O+]
Reversing and re-solving that expression:
[H3O+] = 10-pH
Why do we follow [H+] or [H3O+]? We know from the Kw expression that [H3O+] and [OH-] are related. Depending upon the type of problem, sometimes it's easier to think in terms of [OH-], but [OH-] is also usually a very small number, so it's useful to define an analogous quantity, pOH:
pOH = -log[OH-]
Reversing and re-solving that expression:
[OH-] = 10-pOH
pH and pOH are related by Kw and we can derive that expression from Kw:
Kw = [H3O+][OH-]
-log(Kw) = -log([H3O+][OH-])
If we generalize that “-log” can be replaced by “p”, and remember that log AB = log A + log B, then:
pKw = -log [H3O+] + (-log[OH-]) = pH + pOH
At 25°C, Kw = 10-14, so pKw = 14.

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